1. Field of the Invention
The present invention relates to an integrated circuit for driving liquid crystal capable of adjusting display contrast.
2. Description of the Related Arts
FIG. 1 is a circuit block diagram illustrating a method of adjusting display contrast using a conventional integrated circuit for driving liquid crystal.
Referring to FIG. 1, a liquid crystal panel 101 includes a plurality of segment electrodes and a plurality of common electrodes arranged in a matrix. A segment driving signal and a common driving signal are applied to the plurality of segment electrodes and the plurality of common electrodes of the liquid crystal panel 101, respectively, and light is turned on only at the intersection of the matrix for which the potential difference between the segment driving signal and the common driving signal exceeds a prescribed value.
A liquid crystal driving integrated circuit 102 drives the liquid crystal panel 101 to present a display. In the liquid crystal driving integrated circuit 102, respective connection points of four serially connected resistor elements R1 forming a resistor are connected to terminals 103-107. The terminal 103 receives a reference voltage VLCD0 setting peak values of the segment and common driving signals, and the terminal 107 connects all components of the circuit 102 in common to ground. The potential difference between the reference voltage VLCD0 and a ground voltage Vss is quartered by the four resistor elements R1. The voltages at the terminals 103-107 will be hereinafter denoted as VLCD0, VLCD1, VLCD2, VLCD3, and Vss, respectively. The common driving circuit 108 receives the voltages VLCD0, VLCD1, VLCD3, and Vss to generate the common driving signal. The common driving signal changes between the reference voltage VLCD0 and the ground voltage Vss to turn on light at the liquid crystal panel 101, and changes between the voltages VLCD1 and VLCD3 to turn off light at the panel 101. Therefore, in this case, the common driving signal assumes a xc2xc bias driving waveform. On the other hand, a segment driving circuit 109 receives the voltages VLCD0, VLCD2, and Vss to generate the segment driving signal. When a light is to be turned on at the liquid crystal panel 101, the segment driving signal changes between the reference voltage VLCD0 and the ground voltage Vss in a phase opposite to that of the common driving signal for turning on light. On the other hand, the segment driving signal remains unchanged at the voltage VLCD2 when light is to be turned off at the panel 101. The reference voltage VLCD0 determines display contrast (difference in display between when light is on and off). Therefore, the display contrast of the liquid crystal panel 101 can be optimized by having a variable reference voltage VLCD0 and changing the amplitudes of the common and segment driving signals.
A reference voltage generation circuit 110 applies the reference voltage VLCD0 to the terminal 103. In the circuit 110, a resistor 111 and a variable resistor 112 are connected in series between a power supply voltage Vdd and a ground voltage Vss. An operational amplifier 113 outputs a voltage equal to that present at the connection point between the resistor 111 and the variable resistor 112 as the reference voltage VLCD0. When the impedance of the resistor formed by the four serially connected resistor elements R1 exceeds the load impedance of the liquid crystal panel 101 and the like, the voltages VLCD1-3 are likely to be unsettled. Therefore, the operational amplifier 113 having a small output impedance is used. A resistor may be externally connected between the terminals 103-107 to form a resistor member connected in parallel to the four serially connected resistor elements R1, to thereby reduce the impedance on the side of the serially connected resistor elements R1. The reference voltage generation circuit 110 receives a control signal for changing the value of the variable resistor 112 from an external controller. Thus, the reference voltage VLCD0 is changed under the control of the external controller, to thereby adjust the display contrast of the liquid crystal panel 101.
However, in the circuit arrangement of FIG. 1, the reference voltage generation circuit 110 must be externally connected to the liquid crystal driving integrated circuit 102. Thus, as the circuit 110 includes a great number of elements, it would impede reduction in cost of electronic devices. In addition, ports of the external controller for specific use are dedicated for output of control signals, which would hinder the electronic devices from assuming higher functions.
FIG. 2 is another circuit block diagram illustrating a method of adjusting display contrast using a conventional liquid crystal driving integrated circuit, which attempts to solve the problems of the circuit in FIG. 1. In FIG. 2, the liquid crystal panel 101, the common driving circuit 108, and the segment driving circuit 109 of FIG. 1 are not shown.
In the integrated circuit 201 for driving liquid crystal, the respective connection points of the four serially connected resistor elements R1 are connected to terminals 202-206 for a similar purpose to that described in connection with FIG. 1. The terminal 202 is a power supply terminal receiving the power supply voltage Vdd. A regulator 207 outputs a constant voltage VRF based on the power supply voltage Vdd. An operational amplifier 208 has a positive terminal connected to the constant voltage VRF, a negative terminal connected to a terminal 209, and an output terminal connected to the terminal 206. The value of current IR flowing across the negative terminal of the operational amplifier 208 can be adjusted under the control of an internal controller.
Three serially connected external resistor elements R2, R3, and R4 forming another resistor are connected between the terminals 202 and 206, and an intermediate terminal of the external resistor element R3 is connected to the terminal 209. The serially connected resistor elements R2, R3, and R4 are divided into two parts by the intermediate terminal of the resistor element R3. The resistance of the part consisting of the resistor element R2 and a portion of the resistor element R3 will be denoted as Ra, and that of the part consisting of the remaining portion of the resistor element R3 and the resistor element R4 as Rb.
A voltage VLCD4 can be given by ((Ra+Rb)/Ra)VRF+IRxc2x7Rb. Thus, the value of current IR is controlled by the internal controller to change the voltage VLCD4, thereby adjusting the display contrast of the liquid crystal panel 101.
However, while the liquid crystal driving integrated circuit 201 of. FIG. 2 requires only the resistor elements R2, R3, and R4 as external elements, a ratio of the voltages Ra and Rb would deviate from the expected value because of variation in resistance of the resistor elements R2, R3, and R4, making it impossible to achieve appropriate display contrast. Consequently, the variation in resistance of the resistor elements R2-R4 must be corrected under the control of the external controller, resulting in similar problems to those discussed in connection with FIG. 1.
An object of the present invention is to provide an integrated circuit for driving liquid crystal that requires no external elements and allows adjustment of display contrast.
The present invention has been conceived to solve the above problems. According to a first aspect thereof, the present invention,provides a liquid crystal driving integrated circuit for generating a liquid crystal driving voltage that drives a liquid crystal panel to present a display from respective connection points of a plurality of serially connected resistor elements forming a first resistor. In the liquid crystal driving integrated circuit, a reference voltage applied to one end of the first resistor formed by the plurality of serially connected resistor elements is variable so as to adjust the display contrast of the liquid crystal panel. The above integrated circuit includes a second resistor formed by a plurality of serially connected resistor elements and connected to a power supply, a reference voltage generation circuit having a selection circuit for deriving one of the voltages at respective connection points of the plurality of serially connected resistor elements forming the second resistor, and generating the reference voltage based on an output of the selection circuit, a holding circuit for holding control data applied from an external source to control the selection circuit, and a decoding circuit for decoding the control data held in the holding circuit and generating a control signal to operate the selection circuit.
The above reference voltage generation circuit may include a plurality of gate circuits for deriving one of the voltages at the respective connection points of the plurality of serially connected resistor elements forming the second resistor based on the value of the control signal, and an operational amplifier receiving the voltage derived from the plurality of gate circuits. An output of the operational amplifier is used as the reference voltage.
The above holding circuit may include a shift register for holding control data obtained by serially connecting first and second bit strings, a clock generation circuit for generating a clock signal based on the first bit string, and a latch circuit for latching the second bit string in accordance with the clock signal and supplying the string to the decoding circuit. The control data is applied from an external source, serially connected with address data for determining whether or not the liquid crystal driving integrated circuit receiving the data is to be controlled. The control data can be held in the shift register only when the address data is matched with a predetermined value. A match detection circuit may further be provided between an external input and an input of the shift register to detect a match between the address data and the predetermined value.
According to a second aspect of the present invention, a liquid crystal driving integrated circuit includes a first switch circuit for connecting one end of the first resistor formed by the serially connected resistor elements with a power supply, a second switch circuit for connecting or disconnecting the second resistor formed by the serially connected resistor elements with or from the power supply, and a circuit for enabling or disabling operation of the reference voltage generation circuit. When the reference voltage generation circuit is to be operated, the first switch circuit is turned off and the second switch circuit is turned on. When the reference voltage generation circuit is to be turned off, the first switch circuit is turned on and the second switch circuit is turned off.